Until recently the only operating parameters of the automobile of any real interest to the typical motorist have been his instantaneous speed, miles traveled, and an indication of the essential quantities of fuel supply, engine temperature, and oil pressure. For a great many years, the car manufacturers have made it a near universal practice to supply only the instrumentation to measure and display these quantities in their products.
The advent of a world wide shortage of petroleum fuels, however, coupled with the resulting large increase in gasoline prices, has made the motoring public more conscious of the mileage performance of the automobile. Fuel conservation has become so much a matter of national concern that the Congress has enacted legislation designed to encourage the manufacture of cars with a much better fuel-mileage performance than that previously considered acceptable.
The individual motorist is strongly encouraged to operate his car in a fuel-conserving manner by driving at only moderate speeds, avoiding quick acceleration, etc. He has no convenient way to determine what mileage performance his efforts are achieving, however, and can only resort to the procedure of maintaining a written record of gasoline purchases and odometer readings, and then calculate, from time to time, the average gas-mileage over some interval of time. The procedure is so laborious that most people simply don't bother, particularly since it provides no detailed facts concerning the effectiveness of specific conservation measures.
Clearly the energy crisis has created a situation where the measurement and display of both instantaneous or current and long term average operating fuel economy are fully as important as the traditional measurement of speed and miles travelled. The prior art of instrumentation for the measurement of gas-mileage has serious inherent limitations, however.
Older methods typically involve inserting a sensing device into the fuel supply line that will produce an electrical voltage generally proportional to the time rate of fuel flow, inserting a second device into the speedometer cabling to produce a second voltage generally proportional to the car speed, electrical means to divide the second voltage by the first, and a meter to display the instantaneous value of the quotient. These systems provide no means for integrating fuel flow over a period of time, and cannot provide a measure of fuel-mileage over a trip, for example.
The measurement of fuel flow rate over the range of speed and acceleration conditions typical of normal driving, is particularly difficult to do without adversely affecting car performance, and imposes severe limitations on the accuracy of these older systems. The fuel gauging device most commonly employed in prior art systems uses a float placed as a partial restriction in the fuel stream, and linked mechanically or magnetically to a variable resistor. A voltage generally proportional to float displacement is produced across two terminals of the variable resistor. The force required to overcome friction in the linkage system with sufficient margin to permit reliable measurement of fuel flow, particularly at low consumption levels, is a significant fraction of the kinetic energy of the fuel stream. Moreover, the mass of a practical float and linkage system is large enough so that the acceleration that can be achieved with the limited force available is marginally small. These inherent characteristics of these flow devices tend to adversely affect engine performance.
Another fuel flow measuring device based on prior art, employs a sphere positioned in a vertical, tapered section of the fuel line. The fuel stream pushes the sphere upward in the widening portion of the tapered section, the vertical position depending ideally upon the rate of fuel flow. The sphere, being opaque, intercepts a proportionate amount of a light beam that impinges upon a photo-conductive element. A voltage proportional to vehicle speed is applied to the photo-conductive element, said voltage being derived from a small electrical generator inserted in the speedometer cable system. This flow meter has the advantage of eliminating the mechanical linkages of float type devices and their attendant friction, but the output is only roughly proportional to flow rate, and the ability of the system to resolve small differences in flow rate is very limited. These limitations are inherent, because the sphere cannot be constrained to move precisely along the axis of the tapered section of the fuel line, because the turbulent fuel flow above the sphere also affects the position, because the cross section of the light beam is not of uniform density in any practical optical system and because of non-linearities in the response of the photo-conductive element.
Although the speed responsive sensor in prior art systems is not subject to the same limitations of accuracy as is the fuel sensor, there are definite limits on the linearity of output voltage of practical devices over the range of speeds which must be measured. These various prior art systems, depending as they do upon sensing fuel rate and speed with devices that produce an output supposedly proportional to the quantity which they measure, have severe limitations of accuracy and resolution over the extreme range of conditions under which the automobile typically operates.
The present invention provides a system for the measurement, calculation, and display of both instantaneous and cumulative average fuel utilizing performance in engines including fuel powered vehicles. Both fuel flow and time of operation or distance travelled are sensed by digital methods, counts are accumulated, either for brief time intervals for essentially current or instantaneous information, or over longer intervals for the calculation of trip averages. Calculations are performed by solid state devices incorporated into integrated circuits. The resulting performance values are preferably displayed digitally as numbers, although conversion to a meter display is possible.